Proton-Coupled Reduction of an Iron Cyanide Complex to Methane and Ammonia

In addition to catalyzing N₂-to-NH₃ conversion under ambient conditions,^[1] nitrogenase (N₂-ase) enzymes facilitate the multi-electron reduction of a wide range of other substrates, such as azide, acetylene, carbon dioxide, and nitrous oxide.^[2] Cyanide (CN⁻) is perhaps most noteworthy in this context as it is isoelectronic with N₂ and can be reduced in two-, four-, or six-electron processes to furnish H₂C≡NH, H₃C-NH₂, or CH₄ and NH₃, respectively.^[3] To date there is scant synthetic precedent for the overall reduction of CN⁻ to CH₄ and NH₃, either stoichiometrically or catalytically, by well-defined coordination complexes; beyond N₂-ases, CN⁻ conversion to CH₄ and NH₃ is thus far limited to extracted nitrogenase cofactors, or via reductive electrolysis by Ni electrodes.^{[4, 5]}

We have had an ongoing interest in the study of single-site iron complexes that mediate N₂-to-NH₃ conversion as functional inorganic models of biological nitrogen fixation at the iron rich cofactors of N₂-ases.^[6] We naturally became interested in whether such single-site precursors might functionally model the enzymatic reduction of CN⁻ to liberate CH₄ and NH₃. Such a process is likely to proceed through classic organometallic intermediates^[7] with appreciable Fe-to-C covalency, and this possibility, in addition to the recent discovery that N₂-ases can also mediate Fischer–Tropsch type CO reduction,^[8] motivates functional organometallic model studies.

Herein we report an Fe(CN) complex supported by a tris(phosphine)silyl ligand ([SiP^iPr₃]Fe(CN), 1, Scheme 1) that releases significant amounts of NH₃ and CH₄ on exposure to excess acid and reductant. We also report the isolation and structural characterization of terminal Fe(CNH) and Fe-(CNH₂) complexes that are plausible intermediates of the overall reduction process.

Scheme 1.

Synthesis of Fe(CNH_x) complexes.

Entry into this CN⁻ reduction system was made via the reaction of [SiP^iPr₃]Fe(Cl)^[9a] ([SiP^iPr₃]=[(2-iPr₂PC₆H₄)₃Si]⁻) with a slight excess of either NaCN, K¹³CN, or KC¹⁵N to furnish the three isotopomers of [SiP^iPr₃]Fe(CN) (1) as pink solids. Compound 1 was found to display rich electrochemical behavior (see the Supporting Information) and reduction with Na(Hg) in the presence of 12-crown-4 cleanly afforded the one-electron reduced salt {Na(12-crown-4)₂}{[SiP^iPr₃]Fe-(CN)} (2) as a dark brown solid. Complexes 1 and 2 are paramagnetic with solution spin states of S=1 (μ_eff=2.6 μ_B) and S=1/2 (μ_eff=2.0 μ_B), respectively, observed at room temperature. Anionic 2 additionally displays a quasi-axial EPR spectrum (g_∥=2.23, g_⊥≈2.05) in a 2-MeTHF glass at 20 K. The solid-state structures^[10] reveal trigonal-bipyramidal Fe geometries (τ₅=1.02 and 0.85 for 1 and 2, respectively; see Figure 1 and the Supporting Information) with the CN ligand positioned trans to the apical Si atom. These Fe–CN complexes are nearly isostructural to the similarly-charged Fe–N₂ and Fe–CO complexes supported by the [SiP^iPr₃] platform.^[9]

Figure 1.

X-ray diffraction crystal structures of 1, 3, 4, and 5 with thermal ellipsoids drawn at 50 % probability. Hydrogen atoms (excepting the N-H’s), the BAr^F₂₄ counteranion of 3 and co-crystallized solvent molecules have been removed for clarity. Two independent molecules of 4 are found in the unit cell and only one is shown.

Fe(CN) 1 is readily protonated by the acid {H(OEt₂)₂}-{BAr^F₂₄} (BAr^F₂₄=(3,5-(CF₃)₂C₆H₃)₄B⁻) to afford the cationic hydrogen-isocyanide complex, {[SiP^iPr₃]Fe(CNH)}{BAr^F₂₄} (3) (Scheme 1). Compound 3 adopts an S=1 spin state in solution (μ_eff=2.6 μ_B), complicating the assignment of the acid-derived proton by NMR techniques. Fortunately, this proton is unambiguously located in the Fourier difference map of the solid-state structure of 3 (Figure 1) as a component of a C≡N-H ligand. A hydrogen bond is observed between the isocyanide proton and a co-crystallized molecule of diethyl ether (d(N…O): 2.639(3) Å, ∡](O…H-N): 173(4)°). The KBr IR spectrum of polycrystalline 3 displays very broad absorbances that span the range of 3100 to 1900 cm⁻¹ (Supporting Information), a feature that may arise from the coupling of N–H and C≡N vibrational modes and a continuum of hydrogen bonding interactions in the polycrystalline state.^[11] Overall, the bond metrics of the {Fe(CNH)} unit compare well to those of the few previously characterized examples.^{[7a, 12]} By comparison, we have to date been unable to directly characterize related {Fe(NNH)} species supported by either tris(phosphino)silyl or tris(phosphino)borane ligands despite their presumed intermediacy in N₂-to-NH₃ conversion reactions.^[6]

Further activation of the CN ligand was achieved via double protonation of the anionic Fe(CN) 2. Combining solutions of 2 with 2.5 equivalents of trifluoromethanesulfonic acid (HOTf) in thawing 2-MeTHF affords the cationic iron aminocarbyne complex, {[SiP^iPr₃]Fe(CNH₂)}{OTf} (4) (Scheme 1). Aminocarbyne 4 displays averaged 3-fold symmetry in THF solution, as judged by its ¹H NMR spectrum at 193 K; its EPR spectrum is quasi-axial (g_∥=2.51, g_⊥≈1.98, Figure 2B) with a larger observed g-anisotropy than that of Fe(CN) 2. The solid-state structure of 4 reveals a shortened Fe⁻C bond (1.800(4) Å) relative to those found in compounds 1–3 (1.97–1.91 Å) and is indicative of significant Fe-to-C multiple bond character. While the CNH₂ hydrogen atoms were not directly located in the Fourier difference map, their presence is indicated by close contacts between the CN-derived N atom, the triflate counteranion and a co-crystallized molecule of THF (d(N…O) range from 2.68 Å–3.20 Å). Solid IR spectra of 4 (Figure 2A) also reveal strong absorptions near 3100 cm⁻¹ that shift to 2300 cm⁻¹ in 4-d₂ (prepared from 2 and DOTf) which are assigned to N-H(D) stretching frequencies engaged in strong hydrogen bonding interactions.^[11] Similar features are noted in the recently-described terminal Fe=NNH₂ complex, {[SiP^iPr₃]Fe≡NNH₂}{OTf},^[6c] consistent with their similar structures. While synthetic Fe complexes have previously been shown to support terminal CNH ligands or bridging CNH₂ functionalities,^[13] compound 4 is the first example of a terminal Fe(CNH₂) complex.^[14]

Figure 2.

Pertinent spectroscopic data for Fe(CNH₂) 4 and related complexes. A) Solid IR spectra of 4 (black) and 4-d₂ (gray). B) X-band EPR spectra of frozen 2-MeTHF solutions of 6 (top), 4 (middle) and 2 (bottom) collected at 20 K. C) Zero field, 80 K ⁵⁷Fe Mössbauer spectra of solid 3 (top), ⁵⁷Fe-enriched 4 contaminated with 24 % 3' collected as a frozen 2-MeTHF solution (middle) and ⁵⁷Fe-enriched 6 (bottom) collected as a frozen 2-MeTHF solution.

We next examined reductive cleavage of the coordinated cyanide unit from Fe(CN) 1 (Scheme 2). After canvassing several conditions we found that stirred Et₂O solutions containing 1, 24 equiv of Cp*₂Co and 24 equiv [2,5-Cl₂-PhNH₃][OTf] maintained at −78°C furnished an average of 0.33(4) equiv/Fe of NH₃ upon warming to room temperature overnight. Isotopically-enriched [¹⁵NH₄][Cl] is the sole nitrogen-containing product detected by ¹H and ¹⁵n NMR spectroscopy when ¹⁵N-1 is subjected to these conditions, hence establishing that the NH₃ is derived from the cyanide N-atom. The use of 50 instead of 24 equivalents of both Cp*₂Co and [2,5-Cl₂-PhNH₃][OTf] does not increase the yield of NH₃ (0.31(2) equiv/Fe). Control reactions that replace 1 with [TBA][CN] as a soluble source of CN⁽⁻⁾ do not generate detectable quantities of NH₃, implicating a likely role of the [SiP^iPr₃]Fe platform in the activation of cyanide towards cleavage.

Scheme 2.

Reductive protonolysis of Fe(CN) 1. Ar=2,5-Cl₂-C₆H₃. Refer to the Supporting Information for experimental details.

To ascertain the C-containing product(s) of these reactions, the headspaces were analyzed by gas chromatography (Figure 3) and found to contain 0.41(8) equiv/Fe of CH₄. Exposure of ¹³C-1 to these reaction conditions furnishes ¹³CH₄ as the dominant isotopomer detected by GC-MS (Figure 3C). Finally, replacing [2,5-Cl₂-PhNH₃][OTf] with partially-enriched DOTf as the proton source furnishes a mixture of deuterated methane products with masses up to and including 20 (Figure 2D) as is expected for CD₄. Very little CH₄ is detected (0.007 equiv) when [TBA][CN] is used in place of 1 in these reactions (Figure 2A). Taken together, these analyses indicate that the [SiP_3iPr]Fe fragment facilitates the proton-coupled six-electron reduction of coordinated cyanide to CH₄ and NH₃ in moderate overall yields.

Figure 3.

A) Representative GC-FID chromatograms of sampled headspaces in the reaction of (dashed gray) 1 or (dashed black) [TBA][CN] with 24 equiv Cp*₂Co and 24 equiv [2,5-Cl₂-PhNH₃][OTf ] in Et O, and (black) 5 with 10 equiv KC₈ and 10 equiv HOTf. B) Mass spectrum of CH₄ produced from 1. C) Mass spectrum of CH₄ produced from compound ¹³C-1. D) Mass spectrum of CH_xD_4−x produced from 1, 24 equiv Cp*₂Co and 24 equiv partially-enriched DOTf.

We also studied the stoichiometric reactivity of the Fe(CNH) and Fe(CNH₂) complexes to assess the intermediacy of these species in a cyanide cleavage process. In the absence of additional proton or electron equivalents, Fe-(CNH₂) 4 is unstable in solution (τ_{1/2 (293 K)}=24 min), decaying to a mixture of Fe-containing products that include {[SiP^iPr₃]Fe(CNH)}{OTf} (3') and [SiP^iPr₃]Fe(OTf)^[6c] as readily-identified species. Independent reactions reveal that both NH₃ (0.09(2) equiv/Fe) and H₂ (0.24 equiv/Fe) are released on leaving solutions of 4 to stand overnight. Furthermore, Fe(CNH) 3' slowly converts to [SiP^iPr₃]Fe(OTf) in THF solutions (τ_{1/2 (298 K)}=4 h), presumably with the loss of the CNH ligand. A similar ligand displacement reaction was observed for the hydrazine adduct, {[SiP^iPr₃]Fe(N₂H₃}{OTf}.^[9a] This ligand exchange is likely irreversible under the relevant reaction conditions as free hydrogen isocyanide readily converts to hydrogen cyanide.^[7a] Non-productive decomposition of an Fe(CNH_x) complex may therefore partially account for the moderate yields (< 41 %) of CH₄ and NH₃ formed upon exposure of 1 to excess proton and electron equivalents.

Given the challenges arising from the thermal instability of 4 and 3 in solution, we pursued more robust Fe(CNR₂) species to model the intermediacy of a terminal Fe carbyne complex in the reductive C–N bond cleavage described above. The neutral and diamagnetic dialkylaminocarbyne complex [SiP^iPr₃]Fe(CNMe₂) (5), can be prepared in a one pot reaction via the sequential addition of Na(Hg) and MeOTf to solutions of 1 (82 % yield) (Figure 1). Fe(CNMe₂) 5 is isoelectronic to the previously reported iron siloxycarbyne complex^[9b] and accordingly displays a short Fe-C distance (1.710(1) Å), a long C-N distance (1.328(1) Å), and a down-field resonance (δ=279.6 ppm) in the ¹³C{¹H} NMR spectrum assigned to the carbyne carbon. Complex 5 is readily oxidized by {Cp₂Fe}{BAr^F₂₄} to afford {[SiP^iPr₃]Fe(CNMe₂)}{BAr^F₂₄} (6). The salient spectroscopic features of Fe(CNMe₂)⁺ 6 closely match those of the isoelectronic Fe(CNH₂)⁺ 4 (Figure 2). Compounds 5 and 6 are very stable in solution and in the solid state when stored at room temperature under an inert atmosphere.

Fe(CNMe₂) 5 was found to react with proton and electron equivalents to afford a mixture of CH₄, Me₂NH and small amounts of Me₃N. Whereas Fe(CN) 1 is reduced to CH₄ and NH₃ with Cp*₂Co and [2,5-Cl₂-PhNH₃][OTf] (vide supra), exposure of Et₂O solutions of 5 to these reagents only furnishes small quantities of CH₄ (ca. 0.01 equiv/Fe). Apparently, more reactive proton and electron sources are required for C–N bond scission in the case of 5, and 0.47 equiv of CH₄ are detected when 5 is exposed to 10 equiv of KC₈ and HOTf (Scheme 3). Reactions that employ ¹³C-5 generate ¹³CH₄ as the predominant isotopomer, confirming the carbyne carbon as the source of this hydrocarbon. Analysis of the reaction volatiles obtained when ¹⁵N-5 is exposed to these conditions reveals two detectable ¹⁵n-containing products (see the Supporting Information). Resonances assigned to the N-H protons of [H¹⁵NMe₃][Cl] and [H₂¹⁵NMe₂][Cl] are present at δ=10.84 and 9.05 ppm, respectively, in the ¹H NMR spectrum and display well-resolved coupling to adjacent ¹⁵n- and ¹H nuclei. Comparison of the features present in the ¹H, ¹³C, and ¹⁵n NMR spectra to authentic samples of these ammonium salts solidifies their assignments. [SiP^iPr₃]Fe(OTf) was identified as the major Fe-containing product of these reactions (see the Supporting Information).

Scheme 3.

Reductive protonolysis of Fe(CNMe₂) 5. Refer to the Supporting Information for experimental details.

The organic products of these reactions lend some insight into possible pathways for Fe-mediated cyanide reduction. Upon exposure of Fe(CNMe₂) 5 to proton and electron equivalents, dimethylamine is found to be the dominant (90 %) N-containing product. Its formation is consistent with a mechanism whereby an early cleavage of the cyanide-derived C≡N bond occurs and is followed by the formation of CH₄. Scheme 4 outlines scenarios by which such a process might in principle proceed, invoking unusual terminal carbide intermediates [path (i)] or terminal methylidyne intermediates [path (ii)-to-(iii)]. Such pathways are conceptually related to the distal pathway of N₂ reduction.^[1b] While only observed as a minor product, the NMe₃ product cannot be derived from a distal pathway, and may indicate a competing pathway wherein H-atom equivalents are delivered to the Fe-bound carbyne carbon of 5 without C–N bond rupture [Scheme 4, paths (iv) and (v)]. Conceptually related crossover distal-to-alternating mechanisms have been proposed for Fe-mediated N₂ activation,^[6c] and these scenarios will warrant further consideration in the present context of cyanide reduction. The fact that different reaction conditions/reagents are required for the activation of the cyanide-derived C–N bond in Fe(CNMe₂) 5 and Fe(CN) 1 necessitates caution in mechanistically relating the reaction profile of 5 to generate CH₄ and HNMe₂ (or NMe₃) with that of 1 to generate CH₄ and NH₃. Nonetheless, reactions pathways that bypass C–N bond cleavage, such as that represented by (ν) of Scheme 4, do not appear to be competent in the case of the proton-coupled reduction of 1 as MeNH₂ is not observed.

Scheme 4.

Possible routes to the formation of CH₄ and amine products.

In conclusion, we have disclosed the synthesis of a trigonal bipyramidal Fe(CN) species whose C≡N bond is cleaved upon exposure to proton and electron equivalents. An unusual Fe(CNH₂) complex has been characterized and may serve as a plausible intermediate en route to the CH₄ and NH₃ products derived from this 6-electron/proton C≡N cleavage reaction. This work lends indirect support to the notion that one (or more) Fe centers within the FeMoco (or FeVco) can serve to activate myriad substrates, including N₂, CO and cyanide. The catalytic reduction of cyanide by a synthetic molecular system remains a worthwhile challenge, and the complexes described here serve as promising starting points.